1. Field of the Invention
This invention relates generally to a beam mislanding correcting system for a color cathode ray tube, and more particularly to a system for compensating for the mislanding of electron beams in a color cathode ray tube that results from thermal expansion of a beam selecting structure of the tube.
2. Description of the Prior Art
In a color cathode ray tube having a screen provided with arrays of color phosphors, there is provided a beam selecting structure, in the form of a mask or grille which has apertures or slits therethrough, to allow the electron beams to land only on the respective phosphors of the screen that emit light of selected colors. In such tubes, the impingement of the electron beams on the beam selecting structure generates heat in the tube by which the temperature of the beam selecting structure is increased. This increased temperature of the beam selecting structure causes thermal expansion or distortion thereof with the result that the positions of the apertures or slits are shifted relative to the respective groups of phosphors of the screen. This causes the landing positions of the electron beams on the screen to shift relative to the respective color phosphors, and such so-called mislanding causes deteriorations in color purity. The mislanding of the electron beams is more pronounced near the periphery of the screen than at the center thereof, and particularly becomes a serious problem in the case of wide beam deflection angle tubes.
Various ways have been proposed for compensating for such mislanding of the electron beams. One conventional way is to shift the position of the beam selecting structure in the tube relative to the screen in response to increasing temperature of the beam selecting structure, for example, by mounting the beam selecting structure by means of bimetallic supports. Another existing proposal is to shift the effective beam deflection center in the direction of the tube axis relative to the beam selecting structure, so as to change the incident angle of the beam in passing through the aperture or slit of the mask or grille. One existing way of shifting the effective beam deflection center invokes the use of an auxiliary beam deflection coil in addition to the main deflection coil, with the current supplied to the auxiliary beam deflection coil being varied in response to changes in the temperature of the beam selecting structure so as to vary the effect of the auxiliary beam deflection coil.
However, the above described existing systems have several drawbacks in that they are complicated in construction and expensive, and it is difficult for them to accurately compensate for mislanding over the whole screen area, that is, either insufficient or excess compensation is provided at various portions of the screen.
In copending U.S. Pat. application Ser. No. 329,049, filed Feb. 2, 1973, and having a common assignee herewith, permanently magnetic devices are disposed at several places on or in the tube to produce magnetic fields that change the electron beam paths so as to compensate for the electron beam mislanding. Each permanently magnetic device comprises a permanent magnet partially enclosed in a magnetic shunt structure that is temperature-responsive in the sense that the permeablility of the shunt changes in accordance with changes in temperature. However, the foregoing arrangement is disadvantageous to the extent that the strength of the magnetic fields produced by these permanently magnetic devices cannot be easily controlled or varied once they have been set.
Further, all of the above described existing compensation systems are undesirably affected by variations in the ambient temperature about the tube. The foregoing will be understood from the fact that the temperature of the beam selecting structure is varied in response to changes in the ambient temperature, whereby the apertures of slits of the beam selecting structure are shifted, and the temperature sensitive elements of the compensation system respond to the variations in the ambient temperature to shift the electron beam paths. However, the phosphors on the screen, which is usually made of glass, are also shifted by expansion or contraction of the screen in response to changes in the ambient temperature and, as a result, there may be no change in the positions of the shifted apertures or slits of the beam selecting structure relative to the positions of the shifted corresponding phosphors on the screen. Thus, changes in the electron beam paths in response to variations in the ambient temperature may result in incorrect compensation or mislanding.